CODEWITHSUNDEEP
haskell
Kevin Meredith
Kevin Meredith

Reputation: 41909

Partially Applied Function - Parentheses around 2nd Arg

I implemented an exercise from Learn You a Haskell.

isUpperCase' :: Char -> Bool
isUpperCase' = (`elem` ['A'..'Z'])

As I understand, (elem ['A'..'Z']) is a partially applied function. Calling isUpperCase' 'B' results in calling isUpperCase apply 'B', which results in True.

Note that, without the parentheses, the following compile-time error occurs:

*Main> :l IsUpperCase.hs
[1 of 1] Compiling Main             ( IsUpperCase.hs, interpreted )

IsUpperCase.hs:3:16: parse error on input ``'
Failed, modules loaded: none.

What's the reason for surrounding the partially applied function with parentheses?

Upvotes: 0

Views: 122

Answers (1)

daniel gratzer
daniel gratzer

Reputation: 53881

It's the fact that the function is surrounded by ``'s.

In haskell foo `bar` baz is exactly the same as bar foo baz. It converts prefix function application into an infix operator.

Just like other operators we can use sectioning like (2 *). In this case

(`elem` ['A'..'Z'])
\a -> a `elem` ['A'..'Z']
\a -> elem a ['A' .. 'Z']
flip elem ['A' .. 'Z']

This is just a clever (hacky) way to avoid writing flip. What you have is equivalent to

isUpperCase' = flip elem ['A' .. 'Z']

Upvotes: 3

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